This invention relates in general to the manufacture of plastic air movement devices and, in particular, to the manufacture of molded plastic fans of both the impeller and centrifugal or blower type.
More specifically, this invention relates to the manufacture of dynamically balanced molded plastic fans which may be balanced during manufacture through selective distribution of the plastic material within the mold.
Molded plastic air movement devices such as plastic fans are widely utilized due to lower material and manufacturing costs. These fans have found use in many varied applications such as hand-held dryers, humidifiers, and vehicle air-conditioning systems. Such fans may be sized less than one inch in diameter to in excess of 18 inches, and are used for applications requiring a wide range of rotational speed. Usually these fans are utilized within a size-rotational speed relationship such that the fan must be dynamically balanced to within very accurate limits to minimize noise or prevent failure of the unit.
Since these air movement devices are usually manufactured by an injection molding process, it has been the manufacturer's practice to mold a test fan blade in the mold cavity then test the dynamic balance of the fan blade so produced. Upon determining whether or not the blade is unbalanced, and the locus of this imbalance corrective steps are taken. The usual method to balance the subsequently produced fans has involved machining away material from predetermined areas of the mold surface to increase the weight at these locations by allowing additional plastic to be added to obtain the desired balanced weight distribution. While this method has been satisfactory to obtain balanced fans it is expensive requiring a considerable amount of machining and polishing of the mold cavity. In addition, due to changes in the density of the plastic material the mold must frequently be removed from the machine and corrective grinding to the mold surfaces effected to continuously produce dynamically balanced air movement devices. Such down time of the molding machines is extremely expensive and, therefore, this method of obtaining balanced fans has not been completely satisfactory.
In order to eliminate the problems created by grinding of the mold surface, one attempt to provide a more convenient method for manufacturing dynamically balanced molded plastic fans is described in R. B. Gelbard, U.S. Pat. No. 3,136,001 "MOLD FOR MOLDING DYNAMICALLY BALANCED FANS". This invention provides a distinct advantage over the previous method of manufacturing dynamically balanced molded plastic fans through grinding the mold surface, by providing an adjustable method of varying the quantity of material at predetermined areas of the air movement device. This prior art discloses that a dynamically balanced fan can be produced by selectively adjusting the amount of material at predetermined areas of the mold cavity through adjustable plugs.
While the invention of U.S. Pat. No. 3,136,001 greatly improves the ease of manufacturing dynamically balanced air movement devices, in many instances due to the configuration of the molds used in injection molding and various inserts carried by the mold halves, it is undesirable to adjust or vary the plugs used to effect balance of the unit from the back side of the molds. Such adjustment requires that the mold and/or core inserts must be removed from the molding machine, taking the machine out of production and, therefore, is not entirely satisfactory.